In the papermaking process, a dilute slurry consisting of about 0.3 to 1% papermaking fibers together with a mixture of about 99% water and other papermaking components is ejected at high speed and precision from the slice opening of a headbox onto a moving forming fabric. The fabric is guided and driven by a number of rolls over various drainage boxes and foils which assist in the removal of water so as to leave behind a randomly dispersed, loosely cohesive network or web of papermaking fibers. At the end of the forming section, this web is transferred to the press section, where further water removal occurs by mechanical pressures as the web is conveyed on or between a series of press fabrics through one or more nips. The now self-supporting but still very wet web is then transferred to the dryer section of the papermaking machine where the remaining water is removed by evaporation. The resulting paper product may then be exposed to various treatments before it is then finally wound onto a reel, cut to size and packaged for shipment.
The main functions of the press fabric are to transport water away from the sheet as it passes through the nip while providing uniform support and pressure distribution so that the fabric does not unduly mark the sheet. There are several press fabric constructions in common commercial use today including: seamed flat woven designs, endless woven or tubular type fabrics, and multiaxial fabrics; the latter two may or may not include a seam. Regardless of construction, the press fabric will ideally have adequate void volume and permeability, low compressibility and resist compaction under load. The present invention concerns a multiaxial press fabric which exhibits these properties.
Multiaxial press felts (or fabrics) are well known and have been described, for example, in U.S. Pat. No. 5,360,656 (Rexfelt et al.), U.S. Pat. No. 5,268,076 (Best et al.) and others. In these known multiaxial press felts, the base fabric is assembled from a continuous, flat woven precursor strip whose width is much less than the finished width of the press felt. This assembly is accomplished by spirally winding successive turns of the strip between two separated rolls, such that each turn is canted at a small angle to the intended machine direction (MD) of the completed felt. Each succeeding turn of the precursor strip is bonded along its longitudinal edges to the adjacent strip so that the base fabric possesses a degree of cohesion sufficient to allow it to be removed from the rolls. At this point in the manufacturing process, the base fabric is a continuous tubular loop which is subsequently collapsed and flattened to provide a two-layer structure at whose opposing ends seaming areas are formed. The collapsed double layer structure has an interior, and exterior paper side (PS) and machine side (MS) surfaces (see FIG. 2). One or more layers of a nonwoven fibrous batt are needled onto one or both of the PS and MS surfaces during subsequent process steps.
It is known from U.S. Pat. No. 7,207,355 to Lee (“the '355 patent”) to employ, in a multiaxial press felt, a base fabric weave construction in which a single system of weft yarns is interlaced with two warp yarn systems so as to locate and maintain the individual warp yarns of the first system in a vertically stacked alignment with the yarns of the second system. This unique yarn configuration enables the fabrics to better provide and maintain their void volume in comparison to prior art fabrics lacking this construction because the stacked yarns resist nesting between each other and thus prevent fabric compression. The novel yarn arrangement also allows for the formation of MD oriented, vertically aligned seaming loops where the warp yarns pass from the PS to the MS surface of the fabric at the opposing folded ends where the seam is installed. Fabrics made in accordance with this patent have been found to perform well and are low marking. As disclosed in the '355 patent, the novel fabric construction is comprised solely of monofilament yarns.
Although multiaxial press felts such as those described by Best et al. and Rexfelt et al. have met with great success in the marketplace, a problem common to all such spirally wound fabrics concerns the appearance of interference patterns in their base fabrics. These patterns often occur when the two layers of precursor strips, both having the same weave pattern and yarns, are overlaid upon one another when the tubular loop is collapsed upon itself to form the base during assembly. The patterns produce what is sometimes referred to as the Moiré effect, and manufacturing complications associated with the effect in such fabrics are well known and have been described elsewhere (see, for example, U.S. Pat. No. 7,473,336 to Hawes et al.).
These interference patterns have created problems that adversely impact the mass uniformity of the staple fiber batt which is needled onto the base fabric. The interference patterns are produced because the MD and CD yarns in each layer of the precursor strips are of the same size and spacing, and are oriented at equal but opposite directions to the final intended MD and CD of the fabric. This creates regions of relatively higher and lower yarn densities in the base fabric. When batt is subsequently needled to the base fabric, its ability to adhere (due to fiber entanglement) in areas with lower yarn densities is less than in areas of higher yarn density. When used as a press fabric in the press section of a papermaking machine, felts with uneven base fabric yarn densities will tend to shed batt fibers comparatively more rapidly from areas of low yarn density because there are fewer yarns there to provide anchorage for them. Batt fiber shedding continues during fabric use, and the mass non-uniformity in the press felt becomes more pronounced, eventually causing uneven dewatering of the paper sheet because the press nip(s) are unable to apply constant pressure to the fabric and sheet. Uneven pressure application at the nip also reduces sheet smoothness, which is undesirable. In addition, the operational life for the press felt is shorter than could otherwise be obtained due to this loss of batt fiber (because the fabric may be removed prematurely).